from 30-90 t/ha applied to soil have shown only limited decrease in the

fraction converted into organic matter (Somrnerfeldt et al., 1988).

The negative intercepts in Table V indicate that loss of organic matter

will continue in many of the present cropping systems without adequate

residue return to soil. The amount of residue required to prevent further

loss can be estimated by dividing the absolute value of intercept by the

Table Vl

Increase in Organic C in Soil with Residue Addition (1947-1954)and

Subsequent Decrease after Termination of Residue Input (1954-1WO)'~6

~~

~

Rate of change

g C/kglyr

Material

added'

~~

Rotted manure

Fresh manure

Straw

Green manure

None (NPK)

a

1947- 1954

1954-1970

+ 1S O

-0.51

-0.41

-0.36

-0.21

-0.09

Time required to

return to original

level"

(yr)

~~

+ 1.06

+0.69

+0.43

0

Adapted from Sauerbeck (1982).

Silt loam on loess soil, 865-mm precipitation zone, Germany.

Residue added at 20 t DM/ha/yr for 6 years.

Original level was 13.0 g Cfkg.

20.6

18.0

13.3

14.3

112

PAUL E. RASMUSSEN AND HAROLD P. COLLINS

slope and multiplying by 2.38 to convert organic C to total residue. Residue

returns of 1.7, 5.4, and 4.6 t/ha/yr were required in wheat-fallow regions

of the Pacific Northwest, U.S. receiving 240,416, and 564 mm of precipitation. Much lower residue return was needed each year if cropped annually (1.9 t) rather than to a crop-fallow rotation (4.6 t). The Iowa data

projected that a residue return of 6 t/ha/yr was required to prevent further

organic matter loss for continuous corn in a humid climate.

While residue input can increase organic matter content, continued

input must be sustained. Large applications of residue for 6 years in

Germany increased soil organic C substantially (Table VI). But when

residue addition was discontinued, organic C returned to its original level

within 13 to 2 1 years. It appears from this study that very little of the added

C was incorporated into relatively stable C fractions in soil.

3 . Residue Burning

Luebs (1962), Oveson (1966), and Rasmussen et al. (1980) in the United

States and Dormaar et al. (1979) and Biederbeck et al. (1980) in Canada

addressed the long-term effects of residue burning on cereal grain yield and

soil nutrient content. Most of the early work involved burning for less than

20 years and did not find any reduction in grain yield or soil organic matter

result from lower C inputs and environmental stresses created by management. These stresses include increased soil acidity from ammoniacal fertilizer use, soil erosion that decreases C, N , and other essential nutrients,

and increased soil density, which reduces aeration and water availability.

It is generally assumed that microbial biomass C and activity measurements are correlated with soil organic C because soil biomass depends on

the quantity of degradable C sources present in soil (Adams and Laughlin,

1981). Anderson and Domsch (1989) evaluated this relationship over a

range of soils and Insam et al. (1989) evaluated over different climatic

zones. Both found a correlation but the relationship was complicated by a

number of integrative factors, especially in drier regions. Anderson and

monocultures and crop rotations, respectively. The difference was primarily due to the type and amount of organic C input; soil physical properties (i.e., texture) were of minor consequence. They suggested that monocultures and rotations were comparable with respect to steady state

conditions, but that no universal equilibrium constant could describe the